There have been methods of observing high speed phenomena in which a high speed repetitive signal is sampled at a predetermined interval to step down to a predetermined frequency. For example, in one method a sampling pentode utilizing a thermal electron source for analyzing electrical signals is provided, where the grid of the pentode is normally biased to the cutoff region and a current flows when a negative sampling pulse is applied to the cathode. In another method, diodes are arranged to form a mixer circuit and when a sampling pulse is applied to the mixer circuit, a signal current flows due to diode characteristics. In still another method, an electro-optical crystal is used and sampling is effected on the basis of ON/OFF characteristics of the crystal.
However, all of the foregoing methods suffer performance inhibiting disadvantages. In particular, the sampling pentode method utilizing the thermal electron source exhibits poor frequency response to the sampling signal in view of the thermal electron source. The method utilizing the diodes arranged to form the mixer circuit responds to sampling signals of only about 20 ps due to limitations of the diode response characteristics, and also experiences a large amount of jitter. Further, the method utilizing the electro-optical crystal lacks reliability since the crystal is sensitive to temperature and humidity changes. In addition, none of the conventional methods are capable of reading out the waveform of an electrical signal in two-dimensions.